Structure, Dynamics, and Power Conversion Efficiency Correlations in a New Low Bandgap Polymer: PCBM Solar Cell

Abstract

Molecular packing structures and photoinduced charge separation dynamics have been investigated in a recently developed bulk heterojunction (BHJ) organic photovoltaic (OPV) material based on poly(thienothiophene-benzodithiophene) (PTB1) with a power conversion efficiency (PCE) of >5% in solar cell devices. Grazing incidence X-ray scattering (GIXS) measurements of the PTB1:PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester) films revealed pi-stacked polymer backbone planes oriented parallel to the substrate surface, in contrast to the pi-stacked polymer backbone planes oriented perpendicular to the substrate surface in regioregular P3HT [poly(3-hexylthiophene)]:PCBM films. A approximately 1.7 times higher charge mobility in the PTB1:PCBM film relative to that in P3HT:PCBM films is attributed to this difference in stacking orientation. The photoinduced charge separation (CS) rate in the pristine PTB1:PCBM film is more than twice as fast as that in the annealed P3HT:PCBM film. The combination of a small optical gap, fast CS rate, and high carrier mobility in the PTB1:PCBM film contributes to its relatively high PCE in the solar cells. Contrary to P3HT:PCBM solar cells, annealing PTB1:PCBM films reduced the device PCE from 5.24% in the pristine film to 1.92% due to reduced interfacial area between the electron donor and the acceptor. Consequently, quantum yields of exciton generation and charge separation in the annealed film are significantly reduced compared to those in the pristine film.

On the morphology of polymer‐based photovoltaics

“…GIXD images showed a strong out-of-plane (OOP) (010) reflections for the neat polymer, which markedly contrasts the (010) orientation seen for P3HT and other conjugated polymers used in the solar cells. 117,118 This unique property arises from the rigidity and planarity of the PTBx backbones. Recently, the molecular orientation distribution of PTB7 was determined by polarizing light absorption spectroscopy and GIXD data (DeLongchamp and coworkers.…”

When Function Follows Form: Effects of Donor Copolymer Side Chains on Film Morphology and BHJ Solar Cell Performance

“…This orientation of the π -π stacked structures would favor charge transport across the interface. [ 9 ] In contrast, most other bulk-heterojunction (BHJ)-OPV materials, such as poly-3 hexylthiophene ( P3HT ), have a strong preference for an "edgeon" orientation, in which the π -conjugated polymer backbone planes lie nearly perpendicular to the anode surface. [ 10 , 11 ] The unusual orientational preference of the PTB1 is attributed to the unique zig-zag backbone (Figure 1a ), which disfavors the perpendicular orientation with respect to the substrate plane, as found for linear backbone counterparts such as P3HT .…”

Linear optical properties and electronic structures of poly(3-hexylthiophene) and poly(3-hexylselenophene) crystals from first principles

“…[11] In these applications, the performance of devices is strongly dependent on the thin-film structure and morphology of the donor polymer on substrates. [12,13] Rr-P3HT is one of the family of regio-regular-poly(3-alkylthiophenes) (rr-P3ATs). .…”